Correct, Dr.C, it is.
Some airlines claim that they are not "planning" for use of Discretion, and point to the fact that half-hour turnrounds are entirely feasible, that sector times are sensible, ignoring, for example, the experience that tells you that at certain times of the year you will always get strong headwinds on this sector, that you invariably get poor slots on that route, and that there are baggage loading difficulties at the other destination resulting in half-hour turnrounds being impossible in fact as opposed to the theory.
Airlines are perfectly entitled to plan for the possible on a new route. However, experience must later be taken into account. The use of Captain's Discretion is only available to cover the unforeseen, and only when the crew are, taking all factors into account, able safely to continue the duty.
Any airline that plans (or relies) upon use of Captain's Discretion is planning on a duty that would be illegal in any other circumstances. Any airline that plans on Discretion is an airline that is incapable of planning properly, and is an airline that is inherently unsafe.